1. Field of the Invention
The present invention relates to the modification of battery grids of the type used in lead-acid storage batteries, and more particularly, it relates to a modification of the shape and/or surface finish of the battery grids of a lead-acid storage battery to improve paste adhesion and the service life of the battery.
2. Description of the Related Art
Lead-acid storage batteries typically comprise several cell elements which are encased in separate compartments of a container containing sulfuric acid electrolyte. Each cell element includes are least one positive plate, at least one negative plate, and a porous separator positioned between each positive and negative plate. The positive and negative plates each comprise a lead or lead alloy grid that supports an electrochemically active material. The active material is a lead based material (i.e., PbO, PbO.sub.2, Pb or PbSO.sub.4 at different charge/discharge stages of the battery) that is pasted onto the grid. The grids provide an electrical contact between the positive and negative active materials which serves to conduct current.
The active material of a lead-acid battery may be prepared by mixing lead oxide, sulfuric acid and water. Dry additives, such as fiber and expander, may also be added. The active material paste is then applied to the lead grid. The pasted plates are next typically cured for many hours under elevated temperature and humidity to oxidize free lead (if any) and adjust the crystal structure of the plate. After curing, the plates are assembled into batteries and electrochemically formed by passage of current to convert the lead sulfate or basic lead sulfate(s) to lead dioxide (positive plates) or lead (negative plates). This is referred to as the "formation" process.
The formation efficiency of lead-acid batteries depends to a great extent on the positive plate, in particular, to the extent of conversion of lead monoxide (PbO) to lead dioxide (PbO.sub.2) in the active positive material. The high electrical potential required for formation appears to be related to the transformation of non-conductive paste materials to PbO.sub.2. A low formation efficiency of positive plates requires a high formation charge. Inefficient charging also leads to deficiencies in the resulting batteries assembled with such plates. Typically, the initial capacity (performance) of the battery is low if the battery is not completely formed, requiring additional cycling to reach specific performance values.
The formation process has been studied for some time and it is recognized that a number of variables affect formation efficiency. For instance, it is well known that by increasing the adhesion between the paste mixture and the grid, formation efficiency can be improved. Among other things, the increased adhesion between the grid and the paste provides for improved interfacial contact between the grid and paste thereby improving current flow between the grid and paste. Accordingly, certain efforts to improve battery formation efficiency have focussed on improving the adhesion between the battery grid and paste. For example, U.S. Pat. No. 3,398,024 discloses a method for obtaining better adhesion of battery paste to a lead grid by dipping the grid in a persulfate or perborate solution prior to pasting, and then pasting the grid while it is still wet.
It is also recognized that improved adhesion between battery paste and the grid can increase the service (cycle) life of a battery. It is well known that lead-acid batteries will eventually fail in service through one or more of several failure modes. Among these failure modes is failure due to corrosion of the grid surface. Electrochemical action corrodes the grid surface and reduces the adhesion between the active material and the grid. In most instances, failure of the battery occurs when the grids are no longer able to provide adequate structural support or current flow due to the separation of the active material from the grid. Therefore, there have been efforts to improve the service life of a lead-acid battery by increasing the adhesion of the grid material to the active paste material.
For example, U.S. Pat. No. 3,933,524 discloses a method of increasing the cycle life of a battery wherein antimony is deposited on a lead alloy grid in order to promote adhesion of the active materials to the grid. It is stated that the antimony may be deposited in a number of ways including electroplating, spraying, vapor deposition, sputtering and chemical displacement.
A similar method of extending the cycle life of a lead-acid storage battery is disclosed in U.S. Pat. No. 5,858,575. In this method, a lead-calcium alloy substrate is coated with a layer of a tin, lead-antimony, lead-silver or lead-tin alloy. The layer of metal on the surface of the grid promotes better adhesion of the active material paste to the grid.
Another similar method is described in U.S. Pat. No. 4,906,540 which discloses a method wherein a layer of a lead-tin-antimony alloy is roll-bonded to a grid base formed of a lead-calcium alloy. It is stated that the surface layer of the lead-tin-antimony alloy enables the battery active material to be retained for a long period of time. The increased adhesion of the paste to the grid serves to improve the cycle life of the battery.
Yet another similar method is described in Japanese Patent Publication No. 10-284085 which discloses a method wherein a coating of a lead-antimony-selenium alloy is fused to a lead-calcium-tin alloy strip and the strip is punched and/or expanded to form battery grids. The grids formed by this process are believed to increase battery life.
Still another similar method is described in Japanese Patent Publication No. 11-054115 which discloses a method wherein a pre-coating of a dense oxide is applied to a battery grid in order to improve paste adhesion in the battery pasting process.
Thus, it can be seen that the adhesion between a battery grid and battery active material may affect, among other things, battery formation processes and battery service life. Accordingly, various methods, such as those mentioned above, have been proposed to improve the adhesion between a battery grid and battery active material.
While these methods may provide satisfactory solutions to the problem of inadequate paste adhesion, they do have certain disadvantages. For example, each of these processes requires the incorporation of an additional material into the grid manufacturing process. In certain processes, the grid must be treated with an additional chemical (e.g., a persulfate or perborate solution, or an oxide). In other processes, an additional layer of a dissimilar metal or alloy must be deposited on the grid by chemical (e.g., electroplating) or mechanical (e.g., roll-bonding) means. It can be appreciated that the additional process steps and materials required in these methods can significantly increase the cost of manufacturing the battery grids. As a result, certain battery manufacturers may be reluctant to incorporate these methods into a production facility.
It is apparent that previous attempts at improving paste adhesion have focussed on the compatibility problems between battery paste materials and the alloys or coatings at the surface of the battery grid. Accordingly, proposed solutions to the problems of paste adhesion have involved the application of a dissimilar metal or coating to the grid surface.
However, it has been discovered that another source of the problem of inadequate paste adhesion may be the configuration of the grid. Consequently, the effect of different battery grid making processes on paste adhesion has been further examined.
As detailed above, grids for lead acid batteries provide structural support for the active material therein, and serve as a current collector during discharge and a current distributor during recharge. Known arts of lead acid battery grid making include: (1) batch processes such as book mold gravity casting; and (2) continuous processes such as strip expansion, strip stamping, continuous casting, and continuous casting followed by rolling. Grids made from these processes have unique features characteristic of the process and behave differently in lead acid batteries, especially with respect to the pasting process.
In the book mold casting process, molten lead is poured into a grid mold and cooled to form a grid. The surface of the grid made from book mold casting is somewhat rough and the geometric shape of the cross-section of the grid wires is usually oval with a sharp angle formed at the plane where the book mold closes. Book mold casting is a batch process and its productivity is much lower than other processes that are continuous in nature.
In the strip expansion process, a cast or wrought lead strip is pierced, stretched above and below the strip plane, and then pulled or expanded to form a grid with a diamond pattern. The surface of the wires perpendicular to the plane of the strip is smooth and the cross-section of the wires is rectangular. Stamped grids also have smooth surfaces and a rectangular cross-section in the wires. For continuous casting, the surface of the grid can be rough on the mold side and is smooth on the belt/air side. The geometry of the cross-section of the wires produced by continuous casting can be a triangle, a trapezoid, a section of an arc or a semi-circle, depending on the mold design. If the grids are rolled after casting, the surfaces become smooth and the cross-section of the grid wires become rectangular.
When applying battery paste to a grid, an oval-shaped wire, such as that in a book mold cast grid, allows the paste to flow around the wire. The rough surface and the sharp angle of the wires provide a mechanical graft and interlock of paste particles. Therefore, the contact between the grid and the battery paste is good and the plate is strong. With rectangular wires, on the other hand, it is much more difficult to make good contact between the battery paste and the surface of the wire moving in a direction perpendicular to the direction in which the paste is applied because the flow of paste must change in a 90 degree step. This is analogous to the situation where water flows down a 90 degree cliff, and the surface right below the edge of the cliff is not contacted by the falling water. With a grid wire orientation other than 90 degrees, the change of paste flow is gradual and continuous and therefore, provides better paste coverage. When the battery paste is cured and dried, it will shrink and generate tensile force at the paste/grid interface. The tensile force at the paste/grid wire interface is at a maximum when the wire surface is perpendicular to the grid surface and at a minimum when the wire surface is parallel to the grid surface. As a result, a gap is formed between the grid wire and the paste at the location where the tensile force is the maximum. This type of plate is weak and the paste will fall off easily. Because of a lack of contact between the paste and the grid, a battery made with this type of plate is much more difficult to form, performs poorly in certain reserve capacity tests, and does not exhibit satisfactory cycle life.
Therefore, there continues to be a need in the battery manufacturing field for alternative methods for improving the adhesion of battery paste active material to the battery grid. More particularly, there is a need for a method that can increase the adherence of battery active material to a battery grid produced by a continuous process, such as strip expansion, strip stamping, or continuous casting, without the need for additional materials such as treatment chemicals or metal coatings.
It is therefore an object of the present invention to provide a method that increases the cycle life of a battery by enhancing the adhesion between the battery active material and the battery grid.
It is a further object to provide a method that increases the formation efficiency of a battery by enhancing the adhesion between the battery paste material and the battery grid.
It is yet another object to provide a method that can modify the wires of a battery grid made from a continuous process to mimic the wire shape observed in a book mold gravity cast battery grid so that the paste can flow around the grid wires to improve the plate strength.
It is yet another object of the present invention to provide a method of making battery grids that allows a battery manufacturer to take advantage of a low cost continuous grid making process without the drawbacks associated with inadequate paste adhesion such as reduced formation efficiency and reduced cycle life.